1. Field of the Invention
This invention relates to serial printing and particularly to serial matrix printers in which dot matrix symbols are formed by rastering.
2. Description of the Prior Art
In serial printers of the dot matrix type, one direction of a two-dimensional symbol such as a character is generated by repeatedly sweeping a dot forming means. The second dimension of the character is generated as a result of a continuous relative movement between the dot forming means and the print medium in the direction transverse to the sweep direction. Character definition is obtained by selectively preventing dots from being formed during selected sweeps or portions of sweeps. In an ink jet printer, a dot forming means comprises a jet forming nozzle which projects a stream of field controllable ink drops toward the print medium during said relative motion. The drops are deflected in the first dimension by field deflection means, which is repeatedly rastered during said relative motion in the second dimension. As a result of the relative motion, the characters are slanted from the vertical unless corrected. In a case where printing is to be done in two opposite directions of relative motion with no slant correction, the characters are slanted in opposite directions on successive print lines. This dual slanting presents an undesirable appearance and affects readability.
One form of slant correction is to physically orient the dot forming means and/or the drop deflection means in the case of the ink drop printers at an angle tilted relative to the line of travel and/or the vertical direction. Various methods for achieving this can be seen by reference to U.S. Pat. Nos. 3,651,588; 3,596,276; 3,813,676 and 3,895,386. Another method in an ink jet printer for slant correction is to apply a compensating field which in the case of the U.S. Pat. No. 3,938,163 involves additional electrodes located in advance of the deflection electrodes which are maintained parallel with the direction of relative motion.
In the prior art, slant correction in the characters is provided only when printing in a single direction. Slant correction using the above techniques cannot be readily practiced if it is desired to print dot matrix characters in two directions of relative motion. Consequently, speedrate advantages obtained from bi-directional printing are not available and the undesirable results of having some rows of characters vertical and others slanted or alternate lines of characters slanted in opposite directions may be avoided only by use of special mechanisms or field structures or both.